Acrylic interpolymers for flexographic inks

ABSTRACT

Acrylic interpolymers consisting essentially of, by weight, about 25 to 95 percent ethyl methacrylate, about 5 to 75 percent acrylic monomer selected from the group consisting of n-butyl methacrylate, n-butyl acrylate, 2-ethylhexyl methacrylate and 2ethylhexyl acrylate, and about 0.5 to 10 percent ethylenically unsaturated acid selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid, and having an inherent viscosity of about 0.2 to 0.35 are useful as improved binders for flexographic inks.

United States Patent [191 Zunker Oct. 9, 1973 ACRYLIC INTERPOLYMERS FORFLEXOGRAPHIC INKS [73] Assignee: E. l. du Pont de Nemours and Company,Wilmington, Del.

[22] Filed: Apr. 2, 1971 [21] Appl. No.: 130,536

[52] US. Cl. 260/80.8, 260/17 R, 260/27 R, 260/78.5 R, 260/33.4 R,260/38.8 UA,

[51] Int. Cl. C08f 15/40 [58] Field of Search 260/80.8, 78.5 R

[56] References Cited UNITED STATES PATENTS 3,248,356 4/1968 Snyder260/29.6

3,320,196 5/1967 Rogers 260/27 3,590,118 6/1971 Conrady et a]. 424/19Primary Examiner-Joseph L. Schofer Assistant Examiner-Stanford M. LevinAttorney-Robert E. Patridge [5 7] ABSTRACT Acrylic interpolymersconsisting essentially of, by weight, about 25 to 95 percent ethylmethacrylate, about 5 to 75 percent acrylic monomer selected from thegroup consisting of n-butyl methacrylate, n-butyl acrylate, 2-ethylhexylmethacrylate and 2-ethylhexyl acrylate, and about 0.5 to 10 percentethylenically unsaturated acid selected from the group consisting ofacrylic acid, methacrylic acid and itaconic acid, and having an inherentviscosity of about 0.2 to 0.35 are useful as improved binders forflexographic inks.

8 Claims, No Drawings ACRYLIC INTERPOLYMERS FOR FLEXOGRAPI-IIC INKSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to acrylic interpolymers which are useful as binders forflexographic inks.

2. Description of the Prior Art Flexographic printing is widely used forprinting nonporous substrates such as films, foils and some heaviermaterials used primarily in the packaging industry. Since the ink mustdry by evaporation from the nonporous substrate, the vehicle for the inkmust be a volatile solvent.

In flexographic printing the ink is commonly applied to the substrate byuse of a rubber printing plate wrapped around a suitable cylinder.Accordingly, the volatile solvent used in flexographic inks must also berelatively inert toward the rubber compositions used to prepare theprinting plates. Alcohols have been found to be quite suitable solventsfor this use.

Acrylic polymers have been used heretofore as binders for flexographicinks. Typical acrylic polymers which have been used in this mannerinclude those containing about 70 to 98.6 percent ethyl methacrylate, to30 percent methylacrylate and about 1.4 percent methacrylic acid. Thesepolymers had inherent viscosities ranging from about 0.2 to 0.44.Although these acrylic polymers are the most alcohol-soluble,commercially available acrylic resins, difficulties have beenencountered with regard to their alcohol solubility when used as bindersfor flexographic inks. It is generally necessary that a less polarcosolvent be used to assist in keeping the acrylic binder in solution.Only small amounts of these cosolvents can be used, however, sincelarger amounts would dissolve the rubber printing plate. Due toevaporation losses from the ink, it is necessary that alcohol be addedfrom time to time to maintain the desired printing quality with theresult that the cosolvent generally becomes more and more dilute. Duringthis operation it is not unusual for the ink binder to eventuallyprecipitate from the ink thereby forming a residue with the pigment.Accordingly, it would be desirable to provide a more alcohol-solubleacrylic polymer which is a good binder for flexographic inks.

SUMMARY OF THE INVENTION Acrylic interpolymers have now been discoveredwhich are improved binders for flexographic inks. These interpolymersconsist essentially of, by weight, about 25 to 95 percent ethylmethacrylate, about 5 to 75 percent by weight of an acrylic monomerselected from the group consisting of n-butyl methacrylate, nbutylacrylate, 2-ethylhexyl methacrylate and 2- ethylhexyl acrylate, andabout 0.5 to percent by weight of ethylenically unsaturated acidselected from the group consisting of acrylic acid, methacrylic acid anditaconic acid, said acrylic interpolymer having an inherent viscosity ofabout 0.2 to 0.35.

DETAILED DESCRIPTION OF THE INVENTION The acrylic interpolymers of thisinvention provide an improvement over the acrylic polymers usedheretofore as binders for flexographic inks. These interpolymers provideimproved alcohol solubility and printability while also providing goodcrinkle adhesion, thermal lamination bond strength and adhesive bondstrength for this use. The acrylic interpolymers of this inventionconsist essentially of three components; namely (1) .ethyl methacrylate,(2) an acrylic monomer selected from the group consisting of n-butylmethacrylate, nbutyl acrylate, 2-ethylhexyl methacrylate and 2-ethylhexyl acrylate, and (3) an ethylenically unsaturated acid selectedfrom the group consisting of acrylic acid, methacrylic acid and itaconicacid.

The term acrylic" as used throughout the specification and claims isintended to cover alkyl methacrylates as well as alkyl acrylates andpolymers thereof. The term interpolymer consisting essentially of" isintended to include unspecified ingredients of impurities which do notmaterially affect the basic and novel characteristics of this invention.In other words, this term excludes unspecified ingredients or impuritiesin amounts which prevent the advantages of this invention from beingrealized.

The acrylic interpolymer should contain about 25 to 95 percent by weightof ethyl methacrylate. When less than about 25 percent of ethylmethacrylate is present, the thermal and adhesive bond strengths tend tobe poor. When more than 95 percent of ethyl methacrylate is present, thecrinkle adhesion tends to be poor. Preferably, about 45 to percent ofethyl methacrylate is present.

The second essential component of the acrylic interpolymer is about 5 to75 percent by weight of acrylic monomer selected from the groupconsisting of n-butyl methacrylate, n-butyl acrylate, 2-ethylhexylmethacrylate and 2-ethylhexyl acrylate. Of course, mixtures of thesemonomers may be used. When less than about 5 percent of this monomer ispresent, the crinkle adhesion tends to be poor. When more than about 75percent of this monomer is present, the thermal and adhesive bondstrengths tend to be poor. Preferably, the interpolymer contains about25 to 55 percent of this monomer. The preferred monomer is n-butylmethacrylate.

The third essential component of the acrylic interpolymer is about 0.5to 10 percent by weight of ethylenically unsaturated acid selected fromthe group consisting of acrylic acid, methacrylic acid and itaconicacid. Of course, a mixture of these acids can be used. At least about0.5 percent of the acid component is necessary to provide good adhesion.The interpolymer should not contain more than about 10 percent of theacid component since greater amounts have an adverse effect upon thewater resistance of the binder. Preferably, the interpolymer containsabout 1 to 5 percent of the acid component. The preferred acid isacrylic acid.

The acrylic interpolymer should have an inherent viscosity of about 0.2to 0.35. An inherent viscosity of at least about 0.2 is necessary inorder to impart good durability to the resulting flexographic ink.Viscosities greater than about 0.35 render the flexographic inkdifficult to handle or require a reduction in the solids content of theink whereby resulting in a deficiency of binder. Preferably, theinherent viscosity is about 0.22 to 0.30. The inherent viscositiesreferred to herein are I measured at 25 C. using 25 milligrams of theinterpolymer in 50 cubic centimeters of chloroform.

The method of preparing the acrylic interpolymer of this invention isnot critical. It is most convenient to prepare this interpolymer by thefamiliar solution polymerization technique using a suitable solvent, forexample, an alcohol in which the interpolymer is dissolved for use as aflexographic ink binder. The interpolymer may also be prepared by otherpolymerization techniques such as bulk, suspension, or emulsionpolymerization of the monomers. These processes are all well known inthe art. Suitable polymerization methods are described, for example, byRiddle in Monomeric Acrylic Esters" Rheinhold Publishing Corporation, N.Y. (1954), Chapter IV, and by Schildknecht in Vinyl and RelatedPolymers, JohnWiley & Sons, Inc., N. Y. (1952), Chapter IV. Whenmethacrylic acid is used as the acid component of the copolymer, itshould be added portion-wise rather than all at once since this acidtends to polymerize in a somewhat blocky fashion. Portion-wise additionof itaconic acid is also helpful since this acid also has some tendencyto provide blocky interpolymers. Acrylic acid can be added all at oncesince it tends to provide more homogeneous polymers.

In the preparation of flexographic inks the acrylic interpolymers ofthis invention may be used as the sole binder, or they may be used incombination-with other binder components. Suitable binder componentswhich may be used in combination with the acrylic interpolymers of thisinvention include cellulosics such as nitrocellulose, shellacs,polyamides, alkyds, chlorinated and other rubbers, rosin derivatives,butyrates such as cellulose acetate butyrate, vinyls, andthe like. Whenan auxiliary binder is used in combinations with the acrylicinterpolymers of this invention, most often it is nitrocellulose orcellulose acetate butyrate. The addition of nitrocellulose or celluloseacetate butyrate to the binder system inparts hardness, scuff andabrasion resistance, blocking resistance, and heat-smear resistance. Theamount of these additives can be controlled to provide optimumproperties. As the amount of additive is increased, adhesion andflexibility of the ink are diminished, while the hardness and heat-smearresistance of the ink is improved.

The acrylic interpolymers of this invention are soluble in suitablealcohol solvents such as ethanol, npropyl alcohol, isopropyl alcohol,secondary butyl alcohol, n-dodecyl alcohol, and the like. Experience hasshown that cosolvents are not necessary when the interpolymers of thisinvention are used as the sole binder for the ink. However, dependingupon other additives which may be present in the flexographic ink, thepresence of small amounts of cosolvent may be desirable. Suitablecosolvents for this purpose include ethyl acetate, n-propyl acetate,isopropyl acetate, 2- nitropropane, and the like. When nitrocellulose orcellulose acetate butyrate is used as an auxiliary binder, the solventtypically contains about 50 to 80 percent by weight ethyl alcohol and to50 percent npropylacetate.

In addition to the binder and solvent, flexographic' inks also containsuitable pigment such as Milori Blue, Benzidine Yellow, and the like.The flexographic ink may also contain other additives such as surfaceactive agents and emulsifiers which improve properties such as pigmentwetting and leveling. Flexographic inks are prepared by mixing thecomponents together using a suitable piece of equipment such as a ballmill, colloid mill, kinetic dispersion mill, or the like.

The acrylic interpolymers of this invention provide 1 flexographic inkswhich have good alcohol tolerance,

crinkle adhesion, adhesive bond strength and printability. Theseresinsqalso provide good gloss and odor characteristics.

Flexographic inks are used primarily for printing on films and foils ofnon-porous substrates such as ceIlo-.

phane, polyvinyl chloride, polyvinylidene chloride, polyolefin,polystyrene, aluminum, and the like.,These films and foils may beuncoated, or they may bev coated for subsequent use on heat sealingequipment, or for use in the preparation of laminated films.Flexographic ink prepared from the acrylic interpolymers of thisinvention provide good heat seal properties. For example, laminates canbe prepared from films in which the only heat-seal layer between thefilms is a printed coating of flexographic ink containing the resin ofthis invention.

EXAMPLE OF THE INVENTION The following example illustrating the novelacrylic interpolymers of this invention and their use in flexographicinks is given without any intention that the invention be limitedthereto. All parts and percentages are by weight unless otherwisespecified.

A four-liter resin kettle equipped with an agitator, addition funnel,reflux condenser, thermometer and nitrogen purge was charged with 600 g.of ethyl methacrylate, 400 g. of n-butyl methacrylate, 15 g. of acrylicacid, 2.0 g. of n-dodecyl mercaptan chain-transfer agent and 1,120 g. ofn-propyl acetate solvent. The mixture was agitated and heated to 93 C.and then a solution of 6.0 g. of azobisisobutyronitrile free-radicalinitiator in 400 g. of n-propyl acetate was added at the rate of 5.0 ml.per min. for minutes. The temperature was maintained at 92-94 C. After75 minutes, initiator flow was stopped and the reaction mixture was heldat 92-94 C. for 15 minutes. Then the balance of i chloroform at 25 C.using a No. 50 Cannon-Fenske Viscometer. The isolated resin was solublein both 95 percent and absolute ethyl alcohol, and 4.0 percent solidssolutions were clear and colorless.

The following test was carried out to'determine resin solubilityperformance in a typical flexographic ink formulation (minus pigments)under printing press conditions: A solution containing 50 g. of theabove 40 percent acrylic solution resin and 14.3 g. of nitrocellulose(containing 10.9-11.2 percent nitrogen and having a viscosity ofone-fourth second measured by ASTM Test Method D-30l-50 using a 12.2percent solution of dry nitrocellulose resin dissolved in a solventmixture containing 25 percent denatured (No. 1) 95 percent ethylalcohol, 20 percent ethyl acetate, and 55 percent toluene) in 76 g.n-propyl acetate and 24 g. of denatured (2B) 95 percent ethyl alcoholwas rapidly agitated in an open 8-ounce wide mouth jar. The originalsolution level was marked and evaporative solvent losses were replacedhourly by denatured (2B) 95 percent ethyl alcohol. The solution did notbegin to become hazy or to increase in viscosity until after 22 hours.

The 40 percent acrylic solution resin of this example was diluted to 20percentwith n-propyl acetate and was drawn down on a polyvinylidenechloride coated 3/4 mil cellophane film with a 6 mil bar, air dried andbaked at 100 C. for 5 minutes. Adhesion was tested by applyingcellophane tape to the acrylic resin coated cellophane film with normalfinger pressure and removing the tape by pulling gently at a 90 angle tothe surface. Excellent adhesion was exhibited. Crinkle adhesion of thecoating to the film was measured by momentarily crumpling a 6 X 6 in.sheet of the acrylic resin coated cellophane film in the hand withmedium pressure. The tendency of the polymer film to crack and/or flakeoff was checked. Excellent crinkle adhesion was demonstrated.

Although the invention has been described and exemplified by way ofspecific embodiments, it is not intended that is be limited thereto. Aswill be apparent to those skilled in the art, numerous modifications andvariations of these embodiments can be made without departing from thespirit of the invention or the scope of the following claims.

I claim:

1. An acrylic interpolymer consisting essentially of, by weight, 25 to95 percent ethyl methacrylate, 5 to 75 percent acrylic monomer selectedfrom the group consisting of n-butyl methacrylate, n-butyl acrylate, 2-

ethylhexyl methacrylate, and 2-ethylhexyl acrylate, and 0.5 to 10percent of ethylenically unsaturated acid selected from the groupconsisting of acrylic acid, methacrylic acid, and itaconic acid, saidacrylic interpolymer having an inherent viscosity of 0.2 to 0.35,measured at 25 C. using 25 milligrams of the interpolymer in 50 cubiccentimeters of chloroform.

2. The composition of claim 1 in which the acrylic monomer is n-butylmethacrylate.

3. Th composition of claim 2 in which the ethylenically unsaturated acidis acrylic acid.

4. The composition of claim 3 in which the inherent viscosity is 0.22 to0.3.

5. The composition of claim 1 which consists essentially of 45 topercent ethyl methacrylate, 25 to 55 percent acrylic monomer and l to 5percent ethylenically unsaturated acid.

6. The composition of claim 5 in which the acrylic monomer is n-butylmethacrylate.

7. The composition of claim 6 in which the ethylenically unsaturatedacid is acrylic acid.

8. The composition of claim 7 in which the inherent viscosity is 0.22 to0.3.

UNITED STATES PATENT OFFICE.

CERTIFICATE OF CORRECTION Patent No. 3 9 7 Dat d 061301731 9, 1-973Inventor) C David William Zunker It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

[- Column 2, line 11+, "of" should be or Column 5,

line 16, "is" should be it Column 6, line 10, Claim 3, "Th" should beThe Column 2, line 62, "25 milligrams" should be 250 milligrams Column6, in claim 1, "25 milligrams" should read 250 milligrams Signs d andseals athis 1mm day of May 1971 (SEAL) At be st:

EDWARD I-LFLETCHERJR. V C. MARSHALL 'DANN Atte sting OfficerCommissioner of Patents

2. The composition of claim 1 in which the acrylic monomer is n-butylmethacrylate.
 3. Th composition of claim 2 in which the ethylenicallyunsaturated acid is acrylic acid.
 4. The composition of claim 3 in whichthe inherent viscosity is 0.22 to 0.3.
 5. The composition of claim 1which consists essentially of 45 to 75 percent ethyl methacrylate, 25 to55 percent acrylic monomer and 1 to 5 percent ethylenically unsaturatedacid.
 6. The composition of claim 5 in which the acrylic monomer isn-butyl methacrylate.
 7. The composition of claim 6 in which theethylenically unsaturated acid is acrylic acid.
 8. The composition ofclaim 7 in which the inherent viscosity is 0.22 to 0.3.